Implantable cardioverter defibrillator (ICD) systems are good examples of a battery powered device which requires a relatively high energy, high voltage output. ICD systems attempt to treat cardiac arrhythmias by passing through the heart muscle a cardioversion or defibrillation countershock, depending on the type of cardiac arrhythmia diagnosed. The objective of the cardioversion or defibrillation countershock is to immerse as much of the myocardium as possible within the electrical field generated by the countershock. The countershock is a truncated capacitive discharge of electrical energy that generally ranges from 50 to 750 volts and from 0.1 to 40 joules.
One of the problems common to all ICD systems is generation of the necessary high voltage, large energy countershock from a low voltage battery supply within the device. Almost all current ICD systems which have been approved by the Food and Drug Administration (FDA) utilize a flyback transformer to increase the low voltage of a lithium silver vanadium oxide (SVO) battery to a much higher voltage which is then stored in a high voltage output capacitor to be discharged as the electrical countershock. Examples of this kind of high voltage output circuitry are shown in U.S. Pat. Nos. 4,774,950 and 5,405,363.
While the SVO battery works well in generating the necessary current to power the flyback transformer so as to develop a high voltage charge in a relatively short amount of time, e.g., less than 10-20 seconds, the SVO cells are relatively inefficient power storage devices in terms of their energy density. As a result, it is difficult to decrease the size of an ICD system which utilizes an SVO battery arrangement.
One alternative arrangement which addresses this problem has been described in U.S. Pat. Nos. 5,383,907 and 5,407,444. In this system an intermediate high current output power storage device, such as a rechargeable battery, is used to power the flyback transformer, with the intermediate power storage device being powered from a low current output battery that is more efficient in terms of energy density than traditional SVO cells.
A different approach is described in U.S. Pat. No. 5,369,351 in which the flyback transformer and high voltage charge storage capacitors are eliminated in favor of a high voltage charge storage array comprised of a large bank of switchable, rechargeable low voltage battery cells. While this system has the advantage of eliminating two of the larger components within an ICD system, the technique requires a large number of electronic switches to allow for the batteries to be charged in parallel at a low voltage and then switched into series to be discharged at a high voltage. In this system, at least two electronic switches are required for each rechargeable battery cell. As a result, the construction and control of such a high voltage charge storage array is more complicated than for a traditional flyback transformer and output capacitor system.
Although there are several different existing systems for developing high voltage outputs in ICD systems, it would be desirable to provide an alternative high voltage output system for an ICD system which can result in further reduction of the size of the ICD system and also simplify the design and control of the high voltage output system.